The known photomachinable (“PM”) glass compositions are Fotoform™ (Corning Incorporated) and Foturan® (Schott Glass). By photomachinable is meant that after the glass is covered with a patterned photomask and exposed to ultraviolet radiation (the UV range is approximately 100-400 nm), the glass is then subjected to a specified thermal treatment to form crystallites in the area that was exposed to the radiation, but not in the areas covered by the photomask which blocks the radiation from reaching the glass, and is further subjected to chemical removal of selected material from the glass. The term “photomachinable” may be viewed as a combination of the terms “photolithographicable” and “chemically machinable.” The resulting glass-crystal composite material (that is, the material formed in the exposed or pattern areas of the glass not covered by the photomask) is preferentially soluble in hydrofluoric acid relative to the unexposed glass (that is, glass covered by the photomask and not exposed to the radiation). This permits one to make patterns desirable for a particular application using a conventional photolithographic processes. For example, the patterns can be holes and cavities, designs such as pictures, leaves or flowers, writing such as script or block letters, channels, and other patterns as may be desirable for a particular application. There were numerous commercial applications that utilized these photomachinable materials where the refractive nature of the material along with chemical durability, non-magnetic and electrical insulating properties made it unique. However, when one uses modern lasers some problems have arisen.
Photomachinable glasses are based on photosensitively opacifiable glasses (also called “opal glasses”) having a lithium disilicate and/or lithium metasilicate (also called lithium monosilicate) crystallite phase. Photosensitively opacifiable glasses which do not contain lithium are described U.S. Pat. Nos. 2,515,939, 2,515,941, 2,559,805, 2,651,145, and 2,651,146. Photosensitively opacifiable glasses that do contain lithium disilicate and/or lithium metasilicate are described in U.S. Pat. Nos. 2,515,940, 2,628,160 and 2,684,911. Opal glasses with patterns or designs are formed by masking an appropriate glass composition with a patterned mask, subjecting it to radiation in a photolithographic process and a subsequent heat treatment as briefly described in the preceding paragraph and in detail in 2,515,939, 2,515,941, 2,559,805, 2,651,145, and 2,651,146. In addition, U.S. Pat. Nos. 2,628,160 and 2,684,911 describe the chemical machining of an opalized glass (photosensitive) containing lithium disilicate and/or lithium metasilicate, for example, by use of a dilute hydrofluoric acid solution. The radiation used in the foregoing patents was supplied using ultraviolet emitting lamps, for example, Hg or Hg—Xe arc lamps, which flooded the entire surface (lithographic mask and exposed glass) of the article being irradiated with less than 400 nm radiation. U.S. Pat. No. 2,515,939 indicates that generally the time and intensity of the radiation necessary to produce the desired effect was determined by trial. U.S. Pat. No. 2,515,941 indicates that for a given composition different colors can be developed in the gas with using different radiation exposure times.
In modern processes, the exposure of the PM glass is accomplished using programmed, focused laser writing protocols. The typical lasers that are used in these processes are high powered pulsed lasers operating at 266 nm and 355 nm. These pulsed lasers have an average power in excess of 1 W, the peak power being delivered with each pulse being much higher. It is desirable to be able to control the exposure sensitivity of the glass without adversely affecting the ultimate processing of the glass or the properties of the finished part made using the PM glass. Examples of such properties and processes that should remain unaffected or minimally effected (that is, the effect is not detrimental to the ultimate intended use or product) include the thermal development schedule, the rate of etching, the microstructure, the degree of warping and the surface smoothness of the product. The particular laser exposure (wavelength and power) that is being used will determine the optimum exposure sensitivity level (recall, exposure is an energy process and not an intensity one). Prior art glasses which have a ‘single’ sensitivity are thus not suitably useful industry-wide for different applications due to the differences in laser writing systems used throughout the industry. Consequently, there is a need for tailoring the sensitivity of photomachinable glass compositions, in a reproducible and economical manner, so that they can be used with different laser photolithographic systems